Signs of SIP ALG or Double NAT
Written by Marissa Orsini
Updated at March 12th, 2024
Table of Contents
Scope:
The following steps will show you how to check a call trace if there is SIP ALG or Double NAT is occurring on the network. SIP ALG modifies SIP packets in unexpected ways, corrupting them and making them unreadable. This can give you unexpected behavior, such as phones not registering and incoming calls failing.
Requirements:
- Call Trace (Preferrably Inbound) - Find out how to obtain one here
Inbound Call Trace
- Open the call trace link in any browser
- Search for the second INVITE on the right side of the frame. The INVITE should show the extension@privateIP as shown below. If the public IP address is shown instead, SIP ALG or Double NAT is enabled
- Open the call trace link in any browser
- Click on the first INVITE, then On the right-hand window, check the Contact line. The Contact should show the extension@privateIP as shown below. If the public IP address is shown instead, SIP ALG or Double NAT is enabled
